Are you shutting down all non-essential programs before starting the game, especially anti-virus and anything network related (utility or monitoring programs, etc)?
Can you friend join if you start a new game? If so, can you then load the latest save from your current playthrough?
If a new game does not work, try testing in a new profile without any mods enabled.
Have you tried sending the save to your friend, to see if you could join if he was hosting?
Have either of you tried playing with anyone else since this started?
Have you tried hosting/joining an Arena match?
Have your friend go into Arena mode, create an arena, open the Connectivity menu and check the settings, then exit to the main menu and try joining in the lobby. Alternately, create a multiplayer game lobby, exit and try joining.
Have you tried different ways to join? Other than the Steam lobby and friend invite function, there is also Direct Connection, with a Server ID (which the host can get in the Connectivity Menu after starting a new game or loading a save) or IP address. In the latter case, it can help if the host sets up port forwarding in their router to connect to the PC (ports 23253-23262 and 23243-23252), and then the clients should be able to connect to the external IP address.
LAN may be an option, but since you are not on the same local network, it would require VPN software, such as Hamachi or Tunngle.
If you use Discord, that may also be an option.
Try resetting your network adapters:
- click Start (or Search in Win 10), type cmd into the search box, right click Command Prompt and select run as administrator.
- type the following command and hit Enter
netsh winsock reset
- when that is done, there should be a message 'Winsock reset completed successfully'
- type exit and hit Enter, then reboot your computer.
For more details, see:
https://iihelp.iinet.net.au/Resetting_Network_Adapter_(Winsock_Reset)
If you can not connect with other people, either, try starting the game, switch to windowed mode and lower the game resolution (and maybe check the option to mute the sound volume when the game doesn't have focus), start a multiplayer game, hit Esc, open the Connectivity Menu, optionally disable online multiplayer (if it is enabled) and make sure LAN Connections is enabled (or that can be done in the options before starting a new game).
Next, start the game again from the '..\SteamApps\common\Divinity Original Sin 2\DefEd\bin\EoCApp.exe' program file (doesn't matter if the first instance is started through Steam or the executable), select Join Online Game in the Campaign section, switch to the LAN tab, rather than Online, and try joining.
